Quinacridone, also referred to as 5,12-dihydroquino[2,3-b]acridine-7,14-dione, is a well-known pigment having the formula I ##STR1##
Quinacridone is known to exist in three major polymorphic modifications: the alpha, beta and gamma form.
The red alpha form is disclosed in U.S. Pat. No. 2,844,484; The beta form is disclosed as a magenta pigment in the U.S. Pat. Nos. 4,857,646; 5,229,515 and 5,281,269; and as a strong violet pigment in U.S. Pat. No. 2,844,485.
Several patents describe various forms of the red gamma quinacridone pigments. For example U.S. Pat. No. 2,844,581 describes a bluish red gamma-II, U.S. Pat. No. 3,074,950 describes a yellowish red gamma-I and U.S. Pat. No. 5,223,624 describes another yellowish red, a gamma-III- form, its X-ray-diffraction pattern and essential color characteristics.
U.S. Pat. No. 4,760,144 describes a process for the preparation of a yellowish red gamma-I type quinacridone form by conversion of the red gamma-II type quinacridone, in that the red gamma-II is milled in alcohol/base or premilled followed by an aftertreatment in a solvent.
The present invention relates to a novel gamma-I type quinacridone pigment having a distinguished color characteristic and its preparation by a new finishing or direct pigmentary procedure. In particular, the inventive pigment is characterized by its color space, its x-ray-diffraction pattern, which is a gamma-I type form; its specific surface area, which is typically in the range of 17.+-.6 m.sup.2 /g, and its primary pigment particle size, which is generally in the range of 0.1 to 0.5 .mu.m.
In comparison to the known gamma quinacridone pigments the new gamma form shows a considerably higher chroma and a distinct difference in hue. The higher the chroma of an organic pigment the more valuable the pigment is. A higher chroma pigment is more attractive and offers better and more styling opportunities in combination with other pigments, for example with other organic, inorganic pigments or pearlescent mica pigments. Thus, a difference in chroma can be of considerable commercial importance. Due to the outstanding fastness properties, its excellent rheological properties and unique color characteristics, it is highly suited for use in plastics and coatings applications, particularly in automotive coating systems.